This invention relates generally to rotation sensors and particularly to fiber optic rotation sensors. More particularly, this invention relates to an apparatus and method for processing the signals output from an optical rotation sensor. Still more particularly, this invention relates to an apparatus and method for reducing errors introduced in the output of a closed loop fiber optic rotation sensor caused by oscillations in the feedback signal in the servo loop.
A fiber optic ring interferometer typically comprises a loop of fiber optic material that guides counter-propagating light waves. After traversing the loop, the counter-propagating waves are combined so that they constructively or destructively interfere to form an optical output signal. The intensity of the optical output signal varies as a function of the interference, which is dependent upon the relative phase of the counter-propagating waves.
Fiber optic ring interferometers have proven to be particularly useful for rotating sensing. Rotation of the loop creates a relative phase difference between the counter-propagating waves in accordance with the well known Sagnac effect. The amount of phase difference is a function of the angular velocity of the loop. The optical output signal produced by the interference of the counter-propagating waves varies in intensity as a function of the rotation rate of the loop. Rotation sensing is accomplished by detecting the optical output signal and processing it to determine the rotation rate. In order to be suitable for inertial navigation applications, a rotation sensor must have a very wide dynamic range. The rotation sensor must be capable of detecting rotation rates as low as 0.01 degrees per hour and as high as 1,000 degrees per second. The ratio of the upper and lower limits to be measured is approximately 10.sup.9.
A closed loop rotation sensor feeds a signal indicative of the Sagnac phase shift to apparatus for adjusting the phase or frequency of the counterpropagating waves to null the rotation-induced phase difference between them. The amount that the waves must be adjusted in either frequency or phase to null the Sagnac phase shift indicates the rotation rate of the sensing loop.
Closed loop fiber optic rotation sensors utilizing phase modulators for servo loop closure are attractive due to the ready availability of components such as integrated optics phase modulators. Such phase modulators are effective for providing the desired amount of phase modulation for measuring rotation rates in the required dynamic range. Unfortunately imperfections in known phase modulators cause amplitude modulation and coherent Rayleigh scattering, which cause the servo loop to become unstable for certain rotation rates. In particular, the system becomes unstable for a zero rotation rate. Other rates at which instability occurs depend upon the modulation/demodulation techniques used in processing the output of the fiber optic rotation sensor. When the feedback signal in the servo loop is oscillating, no useful output from the fiber optic rotation sensor is available.
U.S. Pat. No. 4,299,490, issued Nov. 10, 1981 to Cahill et al. describes a phase nulling fiber optic rotation sensor using a frequency shifter in a feedback loop. The frequency shifter is placed at one end of the sensing coil so that both of the counterpropagating waves are shifted in frequency. The frequency shifting of the counterpropagating waves generates a non-reciprocal phase shift that is adjusted to offset any phase shift induced by rotation of the sensing coil. The signal required to null the rotation-induced phase shift is then processed to determine the rotation rate.
U.S. Pat. No. 4,372,685, issued Feb. 8, 1983 to Ulrich discloses a fiber optic rotation sensor that uses the Faraday effect to modulate the phase of counterpropagating waves in the sensing coil to null the rotation-induced phase shift.
U.S. Pat. No. 4,717,256 to Ensley et al. is directed to a fiber optic rotation sensor that includes a phase modulation of the counterpropagating waves in the sensing coil. An oscillator drives a phase modulator and also provides sinusoidal reference and timing signals to signal processing circuits to provide synchronous integration and extraction of the rate phase information from a modulated signal indicative of the interference pattern of the counterpropagating waves. The product of the modulated signal and the reference sine wave is integrated over an integer number of complete reference cycles. Ensley et al. discloses that this integrated signal is directly proportional to the product of the sine of the input angular rate and in inertial space and the first order Bessel function that describes the optical phase modulation. The integrated signal is applied to a sample and hold circuit in which the sampling period corresponds to the integration period. The output of the sample and hold circuit is applied to a balanced driver circuit to provide a rate output signal that is directly proportional to the input angular rate.
U.S. Pat. No. 4,735,506, issued Apr. 5, 1988 to Pavlath describes a fiber optic rotation sensor that includes both a fiber optic frequency shifter and a fiber optic phase modulator for modulating the counterpropagating waves.
Another source of instability in the servo loop is electrical cross talk between the phase modulator drive signal and the photodetector circuits used to convert the Sagnac phase shift into electrical signals.